The present inventive concept herein relates to semiconductor memory devices, and more particularly, to nonvolatile memory devices.
Semiconductor memory devices are classified as volatile memory devices and nonvolatile memory devices. A volatile memory device loses its data when power is interrupted. A nonvolatile memory device maintains its stored data even when power is interrupted. Nonvolatile memory devices may include various types of memory cell transistors, and may be divided into flash memory, ferroelectric random access memory (RAM), magnetic RAM, phase change RAM, and the like, depending on the structure of memory cell transistors.
A flash memory device may be a NOR flash memory device and a NAND flash memory device, depending on the connection state of memory cells and bit lines. The structure of a NOR flash memory device includes two or more memory cell transistors connected to one bit line in parallel. Thus, a NOR flash memory device has a superior random access characteristics. The structure of a NAND flash memory device includes two or more memory cell transistors serially connected to one bit line. This structure is called a cell string structure and each cell string needs a bit line contact. Thus, NAND flash memory devices are more highly integrated.
Memory cells of a flash memory device are divided into on-cells and off-cells depending on distribution of threshold voltage. An on-cell is an erased cell and an off-cell is a programmed cell. A memory cell stores one or more bits data, depending on the type of memory cell. When a memory cell is configured to store one-bit data, the memory cell is programmed in either an erase state or a program state. When a memory cell is configured to store two-bit data, the memory cell is programmed in an erase state or one of three program states. When a memory cell is configured to store three-bit data, the memory cell is programmed in an erase state or one of seven program states.
To increase storage capacity and improve integration of flash memory devices, a flash memory device having a three dimensional structure is being studied.